Hole in joist - question on span

[Sloth]

I need to make a hole in a floor joist - I know it needs to be within 0.25 to 0.4 x the span, and in the middle of the joist etc, but I can't find a clear answer on what the span is when there is a supporting wall in the middle. Is the span seen to be between the outer wall, and the supporting wall in the middle? Thus you have a span either side?

 

[telectrix]

the span is between 2 supporting walls, so in your case you have 2 spans.

 

[buzzlightyear]

I need to make a hole in a floor joist - I know it needs to be within 0.25 to 0.4 x the span, and in the middle of the joist etc, but I can't find a clear answer on what the span is when there is a supporting wall in the middle. Is the span seen to be between the outer wall, and the supporting wall in the middle? Thus you have a span either side?

new build or old build?

 

[Sloth]

Thanks guys.

Old 60s ex-council house. There are holes in various places, both original and added-at-some-point in the past that are very much not in places they should be. I'd rather not add any others where they shouldn't.

 

[Midwest]

I wouldn't worry too much about it.

 

[willy fixit]

Remember sizing of joists is not to do with overall strength but rigidity.
you could halve the size of all the timbers in your house and it would still stay up. Just wobble a bit more... then fatigue and fall down a bit sooner than it would have if you'd left it as the designer intended.
If your hole is for one cable (say 10mm dia.)and if its in the centre go the joist it'll make no difference at all. Or rather the affect will be negligible.

 

[Toneyz]

what are you trying to put through?

 

[ELECNEWT]

Remember sizing of joists is not to do with overall strength but rigidity.
you could halve the size of all the timbers in your house and it would still stay up. Just wobble a bit more... then fatigue and fall down a bit sooner than it would have if you'd left it as the designer intended.
If your hole is for one cable (say 10mm dia.)and if its in the centre go the joist it'll make no difference at all. Or rather the affect will be negligible.

I'm sure you were only exaggerating to illustrate a point, but his house would fall down fairly quickly.

If you halve the width of a beam you reduce its strength by a factor of two
If you halve the depth of a beam you reduce its strength by a factor of eight
If you halve both width and depth you reduce its strength by a factor of sixteen